1. Field of the Invention
The present invention relates to a digital FM demodulating apparatus for demodulating a frequency-modulated (FM) signal to be used for a signal recording/reproducing system such as a video cassette recorder (VCR), video disc player, and the like.
2. Description of the Prior Art
In recent years, for improving the home use so as VCRs to have higher performance, higher function, and more rationalized circuit configuration, attempts have been made to perform the signal processing for recording and reproducing by means of digital signal processing.
With respect to the FM demodulation of the reproduced FM signal of a VCR, a digital FM demodulating apparatus is disclosed in the paper, "DIGITAL SIGNAL PROCESSING IN VIDEO TAPE RECORDERS", by Sonke Mehrgardt, IEEE Trans. on Consumer Electronics, Vol.CE-31, pp374, August 1985.
By the way, the FM signal reproduced from a magnetic tape has suppressed upper sideband signals and emphasized lower sideband signals. This would frequently cause the reproduced FM signal to be over-modulated. Therefore, the signal demodulated by the abovementioned digital FM demodulating apparatus is damaged at the over-modulated portions. As a result, the image produced by the demodulated signal inversely drops into a black level in the portion where the signal should rise up from a black level to a white level, thereby producing a degraded image.
Another problem is that the dynamic range of the demodulated signal becomes extremely large when, even if no over-modulation phenomenon occurs, the lower sideband signal of the reproduced FM signal is relatively large compared to the upper sideband signal. Therefore, the number of bits for expressing the signal must be large, involving a problem of enlargement in the succeeding circuit scale. On the other hand, if amplitude limitation is made by an ordinary amplitude limiter to reduce the dynamic range, the level of the direct current component of the signal would be changed.